Electron discharge tube for ultra high frequencies



1947' J. H. o. HARRIES 2,415,349

ELECTRON DISCHARGE TUBES FOR ULTRA HIGH FREQUENCIES Filed Sept. 4, 19414 Sheets-Sheet l lA/VEA/TOR nrrmA/gg 9 .1. H. o. HARRIES 1 ELECTRONDISCHARGE TUBES FOR ULTRA HIGH FREQUENCIES Filed Sept, 4, 1941 4Sheets-Sheet 2 7" God :ZTTORNE F Feb. 4, 1941.

J. H. o. HARRIES 2,415,349

EEECfIRON DISCHARGE TUBES FOR ULTRA HIGH FREQUENCIES Filed Sept. 4, 19414 Sheets-Sheet 3 ll Ti Fig.3.

V .514 5 J s T J T 1 l II E BL L D2 J. H. o. HARRlES 2,415,39

ELECTRON DISCHARGE TUBES FOR ULTRA HIGH FREQUENCIES Filed Sept. 4, 19414 Sheecs-Sheet 4 Patented Feb. 4, 1947 ELECTRGN DISCHARGE TUBE FOR ULTRAHIGIEFREQUENCKES John Henry Owen Harriea'Clrtphatrh Park, London S, W.4, England Application September 4, 1941, Serial No. 409,587 In GreatBritain September 5, 1940 16 Claims; 1

This invention relates to electron discharge tubes, and moreparticularly to the production of beams of electrons and the control ofthe same in electron discharge tubes; the invention has ap-- plicationto, but is not limited to, electron dis" charge tubes, as set forth inthe specificationsof British Patents Nos. 380,429, 385,968 and 521,l99.

Electron discharge tubes are known in which a beam or jet of electronsis deflected over a series of contacts or the like. Such tubes may beused as relaysor frequency multipliers. Thebeam may be deflected byinput currents or voltages applied, for instance, to electrostaticdeflecting plates or magnetic deflecting coils and the output currentproduced in circuits connected to the contacts upon which the beamimpinges. Pulsating currents or amplified currents maybe produced inthese circuits in accordance with the movement of the beam, which is inturn proportionalto the variation of the deflecting voltages or currentsapplied to the deflectingplatesor deflection coils. Electron dischargetubes of this kind are sometimes known as deflection valves. Oneparticularly important application of the. present invention is theproduction of a beam of electrons for use in a deflection valve.

Certain difficulties arise, particularly if. such valves are operated atvery short wave lengths. One of these is that if the valves are to havea high velocity of beam current (such as will give a desirably shorttransit time), then a fairly high voltage has to be applied along thelength of the beam. It is, however, fundamental to the operation ofthermionic devices, thatif energy is to be withdrawn eficiently fromsuch a beam of .electrons for operation of an output device (e. g., forwireless transmission), then the potential of the output or targetelectrode must be able to, fall to a potential approaching zero withoutsubstantially reducing (in this case) the beam current.

A further problem consists in providinganoutput circuit and outputtarget system which has a very low loss at extremely short wave lengths,but which is also capable of operating eficiently in withdrawing energyfrom the electron beam.

Thus, the present invention in the main aims at providing an arrangementwhich enablesa beam of electrons to be decelerated'during operationwithout substantially reducing the beam current flowing to an output ortarget electrode, and to provide a construction of deflection valvecapable of overcoming generally the above difiicul ties.

The usual method of enablingthe output anode electrode to take up alower potential than that of adjacent electrodes without resulting inthe loss of anode current, is to insert an electrode operated as asuppressor grid between the anode and the said adjacent electrodes. Itis found, however, that at very high frequencies it is extremelydiflicult to design-such a suppressor grid and its associated circuit sothat it" does not acquire a high frequency potential which lies betweenthat of the anode and of the adjacent electrode which thereforepreventscorrect operation of the suppressor grid. Therefore, the presentinvention is also directed to enable efiicient operation to be obtainedwithout inserting. an electrode of the nature of a suppressor grid inthe high frequency field of the anode.

When a-deflection valve is to bev operated on a very short wave length,say below 1 meter, circuits for use for output purposes at suchra veryshort wave length in general consist of an enclosed metallic structure,such as the well-known shielded concentric quarter-wave transmissionline; The losses in such. a transmission line circuit become veryconsiderable if the continuity of its outer conductor or shield isbroken, and it is not easy to devise a construction of valve such as ade fiection valve, by means of which an electron beaminay give energy tosuch an output' circuit, while at the same time preserving a low valueof loss in the circuit. Therefore, the present invention also aims atsolving this dificulty.

According to the present inventioinin anelectron discharge tube inwhicha beam of electrons is deflected ormodulated in order to enablealternating energy to be taken from an output electrode system, thelatter consists of an output electrode or anode and another outputelectrode or sub-anode which is maintained at zero alternating currentpotential and shields the alternating fielcl' of the output anode fromthe other electrodes of thetube, while an accelerating electrode isprovided maintained at a positive potential and mounted further from thecathode than the device used for deflecting or modulating the beam;furthermore, a retarding electrode maintained at a relatively lowpotential is mounted next to the accelerating electrode and between itandthe output system, so that the sub-anode may be set 'at a directcurrent potential lower than that of the accelerating electrode so thatthe potential of the output anode may assume an'instantaneous value,during operation, lower than that of the'accelerating electrode'withoutreduction of current to the output electrode, or appreciable loss offocus of the electron beam This arrangement enables the output system toinclude,

in addition to one Or more output targets or anodes, an electrode orsub-anode consisting of a transverse metallic partition Which dividesthe tube into two parts and is formed with one or more apertures throughwhich the electron beam may pass from the part of the tube containingthe cathode and the beam deflecting or modulating device, to the part ofthe tube in which the output target or targets is mounted. The sub-anodeand the output target or targets may then extend outside the envelope ofthe tube and be fitted directly to an outer shield and a high potentialpole or poles of an output circuit The sub-anode is then maintained at apotential between that of the retarding grid and the mean potential ofthe output target or targets.

The sub-anode referred to and the target or targets are preferably madeof copper sealed into the envelope of the tube, the sub-anode being acopper disc which electrically and mechanically separates the outputtarget from the other electrodes. The output circuit employed may be ofthe shielded concentric line or similar type, which is suitable for veryshort wave operation.

Since the potential of the sub-anode is low compared with the steadydirect potential of the output target or targets, the potential of thelatter may decrease substantially without substantially reducing thebeam current, thus providing for efficient transfer of energy from theelectron beam to the output circuit. At the same time, the transit timeof the beam in that part of the tube in which it is acted upon by thedeflecting or modulating device, is short because of the comparativelyhigh potential on the accelerating electrode. Nevertheless, undesirabletransfer of secondary radiation from the low potential subanode to thehigh potential accelerating electrode is prevented by the insertionbetween them of the comparatively low potential retarding electrode.Furthermore, the electric field of the output circuit may even, in thecase of an extremely short wave length, be confined entirely to thetarget side of the sub-anode, that is to say, if a concentric lineoutput circuit is used, the field is entirely confined to the inside ofthis circuit. This condition, as is well known, is necessary if severelosses are to be avoided in such a circuit. The losses are also very lowbecause the connection of the internal electrodes of the tube to theexternal output circuit themselves form part of that circuit.

The invention may also be combined with that set forth in patentapplication Serial No. 409,588, filed September 4, 1941.

In order that the invention may be more clearly understood and readilycarried into effect, a form of electron discharge tube in accordancetherewith will now be described by way of example with reference to theaccompanying drawings, in which-- Figure 1 is a central longitudinalsection of the tube;

Figure 2 is a so-called exploded view in perspective showing theelectrodes from the deflecting cylinder to the output target in detailbut spaced further apart than they are in the actual tube;

Figure 3 is a longitudinal section at righteangles to Figure 1 andshowing the same electrodes as Figure 2;

Figure 4 is a cross-section of the tube taken on the line IVIV in Figure1;

Figure 5 is a central section showing the mounting of the tube and themechanical details of the output circuit; while Figure 6 is afragmentary section showing in detail a method of securing the tube onits mount- The electron discharge tube illustrated is the same as shownin application Serial No. 409,585, filed September 4, 1941 and thecomplete sequence of electrodes is shown in Figure 1 comprising thespiral cathode C, the negative grid G mounted close to the cathode C,the positive grid G and the deflection electrode of cylindrical shape DD seen in Figures 1 to 3. The electrodes so far mentioned are notspecially concerned with the present invention and need not be furtherdescribed in detail.

However, the next electrode is the high potential electrode L and thenthe retarding electrode or suppressor grid S. These are followed by thesub-anode SA and finally the anode or output target T. The dimensionsand spacings of the electrode C, G G D D are exactly as set forth inapplication Serial No. 409,585.

The electrode L is a disc of molybdenum 0.5 mm. thick and spaced fromthe end of the deflecting cylinder D D by a distance of 0.5 mm. Theelectrode L has an aperture of the shape seen in Figure 2, which is 14mm. across the flats and has a diameter of 16 mm. for the areshapedpart.

The suppressor electrode S is spaced 9. distance of 2.5 mm. from theelectrode L and is a disc similar to the latter but having an aperture0.5 mm. larger all round than the electrode L The lead-in wires to theelectrode L are shown at 9, 9', and that for the suppressor electrode Sat I 0 in Figure 4.

The sub-anode SA is a massive copper disc sealed to the glass of theenvelope E and with its nearest surface spaced 3.5 mm. from that of thesuppressor electrode S. As can be seen in Figures 1 and 3, the sub-anodeSA extends outside the envelope of the tube, and as will be explainedlater, is adapted to be fitted directly to the output circuit. It alsodivides the tube into two parts separating the lower part of the tubecontaining the cathode C from the upper part of the tube containing theoutput target T. The electrode SA has a single rectangular slot 8through which the electron beam passes When it is not deflected. It willbe evident, however, that if more than one output target such as T isprovided, more than one rectangular slot .9 could be provided eachassociated with one of the output targets. In the form shown the slot 3is 4 mm. wide and 20 mm. long. The sealing of the sub-anode SA to thetwo parts of the glass envelope E is effected by means of annularfin-like flanges 5, 5, extending from opposite surfaces of the electrodeSA, and in order to avoid the glass breaking away, the section of theseflanges has a pronounced taper towards the root, but the points are thinwith practically parallel sides.

All of the electrodes except the cathode C and the deflection cylinder DD so far described are each provided with a pair of spaced apertures l,as is also the mica disc m. These apertures are in alignment with twoscrew-threaded holes 8 in the sub-anode SA for use in assembling andaligning the electrodes, as described in Patent Application Serial No.409,589, filed September 4, 1941. This method preserves perfectalignment which is very important indeed since unless it is preserved,very indifferent results are obtained.

The anode or output target T has its front surface spaced 10 mm. fromthe nearest surface of the sub-anode SA. The target is of the shapeshown, being sealed into the glass, as shown in Figure 1, by an annularfin 11, so that its rear surface is accessible from outside the envelopeand is formed with a screw-threaded socket t for receiving the conductorby means of which connection is made to it. As already explained, thetarget T is placed behind the slot 8 in the subanode SA.

As an example of potentials which may be applied to the differentelectrodes, the following may be mentioned. The grid G is at the samepotential as or a few volts negative to the cathode C. The positive gridG is held at 1000 volts positive, The mean potential of the deflectionand focussing cylinder D D is 300 volts. The electrode L is at the samevoltage as the anode T which is 2500 volts. The suppressor electrode Sis at cathode potential and the sub-anode SA is maintained at 1200 to1500 volts positive with respect to the cathode C.

The beam of electrons is drawn from the oathode C by the positivepotential on the positive grid G and passes through the deflecting andfocussing cylinder D D When there are no deflecting potentials, itpasses entirely through the slot 5 in the sub-anode and strikes thetarget T. For the reasons already explained, under the above workingconditions, a focussed beam is produced of rectangular or ribbon shapein crosssection so as to conform to the slot 5, and is consequentlydeflected by the electrode D D in the direction of the smaller dimensionof its crosssection. In this way, a greater sensitivity is obtained anda greater current change .per unit of deflecting force than would be thecase if the beam were deflected in another direction with respect to itscross-section.

It can now be appreciated that since the subanode SA is kept at apotential low compared with the steady potential of the output tar et T,the potential of the latter may decrease substantially withoutsubstantially reducing the beam current and thereby provide an efficienttransfer of energy from the beam to the output circuit. At the sametime, the transit time of the beam in that part of the tube where itpasses through the deflecting cylinder D D is short because of thecomparatively high potential on the positive accelerating electrode L Atthe same time, undesirable transfer of secondary radiation from thesub-anode SA, which is at relatively low potential to the high potentialaccelerating electrode L is prevented by the presence between them ofthe low potential retarding electrode S. Furthermore, even whenoperating on extremely short wave lengths, the electric field of theoutput circuit may be confined entirely to the target side of thesub-anode SA, and if a concentric line output circuit is used, as willbe described with reference to Figures 5 and 6, the field is entirelyconfined to the inside of this circuit which is necessary if severelosses are to be avoided in the circuit.

The mechanical details of the output circuit are shown in Figures 5 and6. The tube is mounted by bedding the sub-anode SA on to a seating on abrass ring Sii with a thin silver ring 3i interposed to make goodcontact. The subanode SA is pressed on to the seating in the ring Ell bythree equally spaced screws 32 (Figure 6) screwed radially into the ring33 and formed with conical points which press the sub-anode SA on to theseating, as can be seen in Figure 6. The voltage of 1500 volts for thesub-anode SA is applied to a ring 38a which is in direct metalliccontact with the ring 3% As already mentioned,

the target T has a screw-threaded socket in its outer or rear surfaceand connection is made to it by a rod l2 screwed into that socket and isactually tubular as shown, and is perforated at its lower end so thatair can be forced in at the top of the tubular rod I2 and can escapethrough the perforations for the purposes of cooling the output targetT. The tubular rod i2, being screwed into the output target T, is fixedrelatively to the brass ring 36. A sheet metal casing 33 open at the topis soldered to a further brass ring 34 secured by screws (t5 to yet afurther brass ring A sheet metal cap 3'! with its lower end slit at 38telescopes into the casing and has vent holes 40a in its top wall it toallow of the escape of the cooling air. The wall Aid is also secured toa tubular sleeve 39 which is capable of sliding on the tubular rod 22.The result of this is that the rod i2, the sleeve as, and end wall ie,and the sheet metal casings 3i and 33, and the brass rings 34 and 3'5,are all subject to the voltage of 2500 volts applied to the outputtarget T. Consequently, hese parts have to be insulated from the rings3% and 39a which receive toe sub-anode voltage of 1500 volts. thispurpose, a sheet of mica it about mm. in thickness is interposed betweenthe rings Etta and 36, but these rings are mechanically fastenedtogether by screws d2 which pass through the ring 36 and through largeholes in the ring 38a and screw into nuts 43 which are ins lated fromthe brass ring 35a by mica washers Thus, the parts subjected to the twodifferent voltages of the target T and subanode SA are insulated fromone another, and the forms the dielectric of the condenser through whichthe two sets of parts are electrically connected. The whole outputstructure will be recognised as a concentric line type of tuned outputcircuit. In order to reduce losses while enabling sufiicient mechanicalstrength to be provided, the casings 33, Si, 16, and the rings t l,should be made or" brass and should be silver-plated since goodcontactsurfaces must'be maintained.

For tuning purposes the metal cap 31 can slide relatively to the metalcasing 33 and to the brass rings which are fixed to the sub-anode SA.For this purpose, a sleeve 45 is secured by set screws 5 .3 to thesleeve 39 and is provided with a righthand screw-thread ll on its outersurface. This screw-thread engages an internal right-hand thread in anut 43 fixed within an ebonite adjusting disc d9. On the other side ofthis disc, there is a nut till having an internal left-hand screwengaged by a threaded sleeve 5| fixed by set screws 52 to the upper partof the rod 12. Now if, for example, the ebonite disc 29 is rotated so asto screw the nut 58 up onthe screw-thread of the sleeve 5! which, ofcourse, is fixed to the rod l2, and therefore also fixed relatively tothe subanode SA, owing to the fact that the sleeves 5| and 45 havethreads of opposite hand, the sleeve 55 will be drawn upwards into thenut 48 at double the rate assuming the pitches of the screwthreads to bethe same. As a result, the sleeve 35 and the sheet-metal cap 3'5 aredrawn up relatively to the metal casing 33. Also, if the disc id isrotated in the opposite direction, the cap ii is forced down into thecasing 33 and by this means the tuning of the concentric conductors i2and 3'! is efiected.

The loop is is a pick-up loop connected to twin-shielded conductors 53for connection to a di-pole transmitting aerial. The loop 55 is a tuningloop connected directly to a small lamp 55 which by its brillianceindicates the tuning point.

I claim:

1. Electronic apparatus comprising an electron discharge tube includingan electron-emitting cathode for producing a beam of electrons, anoutput electrode comprising a target anode for receiving electrons fromsaid beam and adapted to alternately vary above and below a meanpotential, a control electrode mounted between said cathode and saidanode, a high potential accelcrating electrode mounted between saidcontrol electrode and said anode, a retarding electrode mounted betweensaid accelerating electrode and said anode and to be maintained at arelatively low potential with respect to said accelerating electrode,and a shielding sub-anode interposed between said retarding electrodeand said anode, said sub-anode to be maintained at the same alternatingpotential as said cathode and constituting a shield to prevent thealternating field of said anode from influencing the electrodes of saidtube located on the opposite side of said sub-anode from said anode.

2. Electronic apparatus comprising an electron discharge tube includingan electron-emitting cathode for producing a beam of electrons, anoutput electrode comprising a target anode for receiving electrons fromsaid beam and adapted to alternately vary above and below a meanpotential, a beam-deflecting element mounted between said cathode andsaid anode, a high potential accelerating electrode mounted between saiddeflecting element and said anode, a retarding electrode mounted betweensaid accelerating electrode and said anode and to be maintained at arelatively low potential with respect to said accelerating electrode,and a shielding sub-anode interposed between said retarding electrodeand said anode, said sub-anode to be maintained at the same alternatingpotential as said cathode and constituting a shield to prevent thealternating field of said anode from influencing the electrodes andelements of said tube located on the opposite side of said sub-anodefrom said anode.

3. An electron discharge tube comprising an electrode system includingan electron-emitting cathode for producing a beam of electrons, anoutput electrode system including a target anode for receiving electronsfrom said beam and adapted to alternately vary above and below a meanpotential and a shielding sub-anode to be maintained at zero alternatingcurrent potential and being formed as a transverse metallic partitiondividing the space within the envelope of the tube into two parts onecontaining the oathode, and one the anode and having an aperture throughwhich the beam of electrons may pass from the space containing saidcathode to the space containing said anode and a control electrodemounted between said cathode and said output electrode system, saidanode and said subanode having surfaces outside the envelope of the tubeadapted to fit mechanically two poles of an output circuit. 7

4. An electron discharge tube comprising an electrode system includingan electron-emitting cathode for producing a beam of electrons, anoutput electrode system including a target anode for receiving electronsfrom said beam and adapted to alternately vary above and below a meanpotential and a shielding sub-anode to be maintained at zero alternatingcurrent potential and being formed as a transverse metallic partitiondividing the space within the envelope of the tube into two parts onecontaining the cathode, and

one the anode and having an aperture through which the beam of electronsmay pass from the space containing said cathode to the space containingsaid anode and a beam-deflecting element mounted between said cathodeand said output electrode system, said anode and said subanode havingsurfaces outside the envelope of the tube adapted to fit mechanicallytwo poles of an output circuit.

5. An electron discharge tube comprising an electron beam-forming systemincluding an electron-emitting cathode, a control electrode for theelectron beam, a high potential accelerating electrode mounted on theside of said control electrode remote from said cathode, a retardingelectrode to be maintained at a relatively low potential with respect tosaid accelerating electrode and mounted next to said acceleratingelectrode and on the side therefor remote from said cathode, and anoutput electrode system including a target anode for receiving electronsfrom said cathode and adapted to alternately vary above and below a meanpotential and a shielding subanode mounted immediately next to saidretarding electrode and between same and said anode and having anaperture through which part of the electron beam may pass to said anodeon its way from said cathode, said sub-anode also to be maintained at apotential between that of said retarding electrode and the meanpotential of said anode.

6. An electron discharge tube comprising an electron beam-forming systemincluding an electron-emitting cathode, a beam-deflecting element forthe electron beam, a high potential accelerating electrode mounted onthe side of said beamdefiecting element remote from said cathode, aretarding electrode to be maintained at a relatively low potential withrespect to said accelerating electrode and mounted next to saidaccelcrating electrode and on the side thereof remote from said cathode,and an output electrode system including a target anode for receivingelectrons from said cathode and adapted to alternately vary above andbelow a mean potential and a shielding sub-anode mounted immediatelynext to said retarding electrode and between same and said anode, andhaving an aperture through which part of the electron beam may pass tosaid anode on its Way from said cathode, said subanode also to bemaintained at a potential between that of said retarding electrode andthe mean potential of said anode.

7. An electron discharge tube comprising an envelope consisting in partof vitreous material, an electrode system including an electron-emittingcathode for producing a beam of electrons, an output electrode systemincluding an anode of copper for receiving electrons from said beam andadapted to alternately vary above and below a mean potential, said anodebeing sealed into a vitreous portion of said envelope and presenting asurface outside said envelope, and a shielding sub-anode in the form ofa transverse copper partition dividing the space within said envelopeinto two parts, having an aperture for afiording the electron beamaccess to said anode and sealed into a vitreous portion of said envelopeand extending outside said envelope, and a beam-deflecting elementmounted between said cathode and said output electrode system.

8. An electron discharge tube installation com prising an electrodesystem including an electronemitting cathode for producing a beam ofelectrons, a target anode for receiving electrons from said beam andadapted to vary alternately above and below a mean potential, ashielding sub-anode in the form of a transverse metallic partitiondividing the space within the envelope of the tube into two parts onecontaining the cathode, and one the anode and having an aperture throughwhich electrons from said beam pass to said anode and having an annularportion extending to the outside of said envelope, a concentric linehaving an inner conductor connected with said anode and a tubular outerconductor surrounding that part of the tube envelope which includes saidanode and being electrically connected with the outer annular portion ofsaid sub-anode.

9. An electron discharge tube installation according to claim 8 whereinsaid sub-anode is to be maintained at zero alternating current potentialand at a mean direct current potential lower than the mean potential ofsaid anode.

10. An electron discharge tube comprising an electrode system includingan electron-emitting cathode for producing a beam of electrons, anoutput electrode system including an anode for receiving electrons fromsaid beam and adapted to alternately vary above and below a mean po.tential and a shielded sub-anode to be maintained at the alternatingpotential of said cathode and interposed between said anode and theremaining electrodes of the tube, a beam-deflecting element mountedbetween said cathode and said output electrode system, an acceleratingelectrode mounted further from said cathode than said beam-deflectingelement and a retarding electrode the form of a disc with a centralaperture for the passage of the electron beam and to be maintained at arelatively low potential with respect to said accelerating electrode andmounted next to said accelerating electrode and between same said outputelectrode systern.

11. An electron discharge tube comprising an electron beam-formingsystem including an electron-emitting cathode, a control electrode forthe electron beam. an accelerating electrode mounted on the side of saidcontrol electrode remote from cathode, a retarding electrode in the formof a disc with a central aperture for the passage of the electron beamand to be maintained at a relatively low potential with respect toaccelerating electrode, and mounted next to said accelerating electrodeand on the side thereof remote from said cathode, and an outputelectrode system including a target anode for receiving electrons fromsaid beam and adapted to alternately vary above and below a meanpotential and a shielding sub-anode mounted immediately next to saidretarding electrode and between same and said anode and having anaperture through which part of th electron beam may pass to said anodeon its way from said cathode, said sub-anode also to be maintained at apotential between that of said retarding electrode and the meanpotential of said anode.

12. An electron discharge tube installation comprising an electrodesystem including an electron-emitting cathode for producing a beam ofelectrons, a control electrode for the electron beam, anoutput electrodesystem including an anode for receiving electrons from said beam andadapted to alternately vary above and below a mean potential and havinga portion extending outside of the envelope of the tube and a subanodein the form of a transverse metallic partition dividing the space withinthe envelope of the tube into two parts and having an aperture to affordthe electron beam access to said anode and extending to the outside ofthe said envelope, a retarding electrode in the form of a disc with acentral aperture for the passage of the electron and to be maintained ata relatively low potential with respect to said anode and mountedbetween said control electrode and said output electrode system, and anoutput circuit including a concentric line having a tubular outerconductor surrounding that portion of the tube envelope which containssaid anode and being electrically connected with said sub-anode, saidconcentric line including an inner conductor electrically connected withsaid outer surface of said anode.

13. An electron discharge tube comprising an electrode system includingan electron-emitting cathode for producing a beam of electrons, anoutput electrode system including an anode for receiving electrons fromsaid beam and adapted to alternately vary above and below a meanpotential and a shielding sub-anode to be maintained at the alternatingpotential of said cathode and interposed between said anode and theremaining electrodes of the tube, a beam-deflecting element mountedbetween said cathode and said output electrode system, an acceleratingelectrode in the form of a disc with a central aperture for the passageof the electron beam and mounted further from said cathode than saidbeam-deflecting element and a retarding element in the form of a discwith a central aperture slightly larger all round than the aperture insaid accelerating electrode for the passage of the electron beam and tobe maintained at a relatively low potential with respect to saidaccelerating electrode and mounted next to said accelcrating electrodeand between same and said output electrode system.

14. An electron discharge tube comprising an electron beam-formingsystem including an electron-emitting cathode, a control electrode forthe electron beam, an accelerating electrode in the form of a disc witha central aperture for the passage of the electron beam and, mounted onthe side of said control electrode remote from said cathode, a retardingelectrode in the form of a disc with a central aperture slightly largerall round than the aperture in said accelerating electrode for thepassage of the electron beam and to be maintained at a relatively lowpotential with respect to said accelerating electrode and mounted nextto said accelerating electrode and on the side thereof remote from saidcathode and output electrode system including a target anode forreceiving electrons from said beam and adapted to alternately vary aboveand below a mean potential and a shielding subanode mounted immediatelynext to said retarding electrode and between same and said anode andhaving an aperture through which part of the electron beam may pass tosaid anode on its vay from said cathode, said sub-anode also to bemaintained at a potential between that of said retarding electrode andthe mean potential of said anode.

15. An electron discharge tube installation comprising an el ctrodesystem including an electron-emitting cathode for producing a beam ofelectrons, a control electrode for the electron beam, an outputelectrode system including an anode for receiving electrons from saidbeam and adapted to alternately vary above and below a mean potentialand having a surface outside the envelope of the tube and a shieldingsub-anode in the form of a transverse metallic partition dividing thespace within the envelope of the tube into two parts and having anaperture to afford the electron beam access to said anode and extendingto the outside of the said envelope, an accelerating electrode in theform of a disc with a central aperture for the passage of the electronbeam and mounted further from said cathode than said control electrode,a retarding electrode in the form of a disc with a central apertureslightly larger all round than the aperture in said acceleratingelectrode for the passage of the electron beam and to be maintained at arelatively low potential with respect to said accelcrating electrode andmounted next to said accelerating electrode and between same and saidoutput electrode system.

16. Electronic apparatus comprising an electron discharge tube includingan electron-emitting cathode for producing a beam of electrons, anoutput electrode comprising a target anode for receiving electrons fromsaid beam and adapted to alternatel vary above and below a meanpotential, an apertured shielding sub-anode positioned in front of saidanode and to be maintained at zero alternating current potential and ata direct current potential lower than the mean potential of said anode,and an apertured suppres- 12 sor electrode positioned in front of saidsubanode and to be maintained at a lower potential than said sub-anode.

JOHN HENRY OWEN HARRIES.

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